Partial characterization of Nocardia farcinica beta-lactamases

The beta-lactamases obtained from culture supernatants and cell extracts of 26 clinical strains and 5 reference strains of Nocardia farcinica were partially characterized. The enzymes exhibited two patterns on isoelectric focusing (IEF). beta-Lactamases from the majority of the 31 strains (87%) including the 5 reference strains exhibited two major bands with pIs of 4.56 and 4.49. The remaining strains had two similar major bands but with slightly higher pIs. Culture supernatants and cell extracts exhibited identical patterns. The two sets of enzymes were functionally indistinguishable by substrate and inhibitor profiles and lack of inducibility. By disk testing, ampicillin, amoxicillin, ticarcillin, amoxicillin-clavulanic acid, and imipenem were highly synergistic with cefotaxime. The enzymes were primarily penicillinases and hydrolyzed cephalosporins at rates of < or = 12% of those for penicillins. N. farcinica beta-lactamases were susceptible to inhibition by clavulanic acid and BRL 42715, exhibiting 50% inhibitory concentrations of 0.025 to 0.045 micrograms/ml (0.12 to 0.22 microM) and 0.05 to 0.1 micrograms/ml (0.31 to 0.63 microM), respectively, less susceptible to tazobactam, and least susceptible to sulbactam, cloxacillin, and imipenem. The beta-lactamases of N. farcinica are believed to mediate penicillin resistance and may play a secondary role in extended-spectrum cephalosporin resistance. The close similarity among N. farcinica beta-lactamases and their distinct differences from beta-lactamases of other Nocardia species support the taxonomic identity of this species.